Understanding the precision engineering aspects of lithium ion batteries

Since their commercial application by Sony in 1990, lithium ion batteries have progressively replaced other rechargeable batteries such as nickel metal hydride batteries, which in the 1990s was the main power pack for remote controlled cars etc. Compared to other types of rechargeable batteries based on different chemistries, the lithium ion battery has the highest energy density with a high charge to weight ratio, and most importantly, with improvements in engineering over the years, is one of the most reliable.

 

However, there remains inherent dangers with lithium ion batteries use. Specifically, damages to any part of the battery such as the thin membrane separating the two electrodes of the battery, or the compartment that prevents the leakage of electrolyte mediating the conduction of lithium ion between the electrodes may result in short circuits or fire. In addition, lithium ion batteries are also vulnerable to overheating induced explosions.

 

Choice of batteries is largely a selection between different battery chemistries as batteries have similar designs. In the case of lithium ion batteries however, the electrolyte ferrying lithium ions (or more technically, providing the liquid medium for the diffusion of lithium ion between the two electrodes) must be carefully chosen with both performance and safety in mind. What are the performance measures? These include: (i) number of recharges without loss of capacity, (ii) speed of charging, and (iii) capacity of battery expressed in mAh (milli ampere hour) or Wh (watt-hour).

 

But why can’t we improve the safety of lithium ion batteries despite gaining increasing operational experience with the battery and its chemistries as well as understanding its manufacturing processes? The problem lies with the precision engineering required in battery manufacture. Specifically, it needs to be manufactured to very small fault tolerance (i.e., all measurement and quality control parameters must be tuned to small ranges) due to the inherent safety concerns of lithium ion batteries, most significant of which is spontaneous ignition during operation, charging or after impact. Precision engineering requires high quality tools for manufacturing, but these may not be easily available to the many contract battery manufacturers which produce inexpensive lithium ion batteries to satisfy consumer demand for cheaper smartphone and tablets. In essence, the economics of consumer electronics place exacting demands on lithium ion battery manufacturing: how to produce a high quality and safe battery at a low price?

In addition to manufacturing tolerances of the battery, material choice is another key factor affecting battery safety. But why must lithium ion battery be encapsulated within a strong material to protect against impact? Because, if the polymer compartment that protects against electrolyte leakage is punctuated, there is heighten risk of spontaneous fire or explosions given the high flammability of the electrolyte. More importantly, given the precision engineering content of lithium ion battery, its production needs to be carefully monitored with quality control a top priority. For example, lithium ion batteries are highly vulnerable to impact (which may cause leakage of flammable electrolytes) and high temperature (i.e., spontaneous ignition of lithium ion battery due to overheating). Thus, various parts of the battery housing needs to be manufactured under high quality control to ensure fault tolerances are within the bounds of safety for the entire battery unit. Specifically, the external plastic housing of the battery should not be so malleable as to transmit shockwaves to the inner electrolyte compartment.

 

Finally, many top end smartphones and tablets comes with a fast charging feature. Is that a problem? My personal experience with budget smartphone and tablets is that they charge slower and are quite safe. Perhaps, in our quest for ever shorter charging time, we may have placed safety at a lower priority. Pumping more electrons into a battery over a shorter period of time may affect both battery construction (i.e., material degradation with charging), and change, in fundamental ways, battery chemistry; specifically, pushing lithium ion chemistry to an unstable state more vulnerable to impact and high temperature induced explosions or fires.

 

In summary, lithium ion battery technology is by far the highest energy density energy storage medium amongst commercialized portable battery technologies. However, the recent advent of fast charging technology may have pushed lithium ion battery technology to an operational region not as safe as slow charging. With vulnerability to impact and high temperature induced explosions, construction of the battery housing is an art of precision engineering utilizing specialized tools, which meant that quality control of the manufacturing process is critical in delivering safe batteries to the consumer. Focus on high quality precision engineering meant that there is a continuum, in quality, of batteries in the consumer market, ranging from ones suitable only for slow charging, and high end models of greater capacity (such as 4000 mAh compared to 2000 mAh) capable of being charged at high rates. But, the perpetual question of the battery industry remains: how to streamline engineering to reduce cost of production and ensuring manufacturing processes remain capable of delivering high quality and safe batteries at affordable price to consumers increasingly demanding higher battery capacity and shorter charging times?

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